Magnetite corrosion products from carbon steel components in the primary heat transport system deposit on the walls of the steam generator tubes in nuclear power plants during operation. The function of the steam generator is to produce steam to turn turbines that generate electricity. Deposits on the walls of the steam generator tubes have a deleterious effect on heat transfer and flow, reducing steam generator performance. As the solubility of iron decreases with temperature, magnetite build-up is generally highest in the cold leg side of the tubes due to lower temperatures in that region. If the magnetite deposits are not removed they will eventually lead to the units being derated.
One known method for removing magnetite deposits from steam generator tubes uses a process akin to sandblasting for removing rust from metal surfaces. Stainless steel spheres of about 100 to 300 μm in diameter are employed as the blasting media. A manipulator system is placed on the cold leg side in the steam generator bowl (also referred to as the primary header or boiler cavity). The manipulator has a blasting head that attaches to one or two of the tube openings and the blasting media is forced through the tubes by compressed air. Blasting media and released magnetite deposits are collected by a second manipulator system on the hot leg side of the primary header. This second manipulator system has a collection head mated to the tube(s) being blasted from the cold leg side. While this system is effective in sealing to the tubes and preventing deposits and blasting media from being released and contaminating the equipment, it is relatively complicated, time consuming and required constant skilled operator attention and precise indexing of the collector head to the tube(s) being cleaned.
Another known method of collecting deposits and media is disclosed in U.S. Pat. No. 6,308,774 which issued to Siemens Aktiengesellschaft on Oct. 30, 2001. This patent discloses a method of cleaning heat exchanger tubes and a collection device for the collection of deposits from heat exchanger tubes. A funnel-shaped collecting vessel that is capable of being folded, rolled or collapsed is introduced through a service orifice or manway opening (usually approximately 18″×14″) into the steam generator cavity and then unfolded, unrolled or opened such that its inlet orifice covers essentially all of the tube ends in the hot leg area of the heat exchanger tubesheet. The collecting vessel, called a “suction header” has an inflatable hose around the inlet orifice which when inflated, expands the inlet orifice to conform to the geometry of the area to be sealed. The system disclosed also includes a device for shaking the suction header to facilitate the removal of waste and debris. The system disclosed in U.S. Pat. No. 6,308,774 has a number of disadvantages.
Firstly, the sealing between the suction header and the heat exchanger tubesheet is inadequate, particularly for minute particles such a magnetite, and unacceptable levels of contamination have been experienced in the field.
In addition, the design of the suction header is such that due to the high velocity of the cleaning media, the very fine magnetite debris is redirected by the suction header and end up passing back through the tubes to the blasting side, thereby contaminating the cleaned tubes and the blasting equipment on the cold leg side of the primary header. Not only is the manipulator exposed to contamination, the minute magnetite particles can escape the boiler cavity, contaminating the immediate environment around the steam generator.
Magnetite particles and grit containing moisture can flock and adhere to the suction header wall. If a large buildup occurs, the flexible suction header can sag due to the weight of the debris, which can compromise the seal between the suction header and the heat exchanger tubesheet. In addition, the suction point can easily become clogged because it is upward facing. This may necessitate the removal and replacement of the suction header, potentially exposing workers to an unnecessary radiation dose. Accordingly, to prevent its occurrence, personnel are required to periodically physically shake the debris from the suction header by inserting their hands into the manway.
The suction header is also difficult to install. To effectively cover all of the tubes in the steam generator and also be able to withstand the high blast force and abrasion of the jet emerging from the tubes, the suction header has to have considerable mechanical strength and is typically manufactured of a relatively heavy thick-walled elastomeric material and takes two strong workers to install. Because it is just slightly smaller that the manway opening, it requires training and skill to insert into the manway and install inside the steam generator. Once installed, adjustments are required to ensure that the peripheral opening of the suction header seals properly to the edge of the tubesheet.
Accordingly, there remains a need for an improved collection system for steam generator cleaning of blasting media and deposit material which overcomes the problems of known systems.